The present invention relates to a document reader for use with a digital copier or similar image recording apparatus and, more particularly, to a document reader capable of reading a document which may be either transmissive or reflective.
A document reader used with a digital copier, for example, customarily includes a reference member having predetermined density which is located in close proximity to one end of a glass platen adapted to load a document. A reflection from the reference member is routed through optics to a CCD (charge coupled device) array or similar image sensor and sensed by a part of the image sensor to be converted into an electric signal. This electric signal is used to stabilize the voltages of a video signal which is outputted by the image sensor with respect to the subscanning direction. More specifically, the voltages of a video signal are stabilized despite any change in the output light of a light source such as a fluorescent lamp in the subscanning direction. With some modern document readers, an extra light source device is available for exclusively reading a transmissive document in distinction from the above-mentioned reflective document. A problem with this kind of scheme is that when the exclusive light source device is mounted on a document reader to read a transmissive document, it is impossible to stabilize the voltages of a video signal because the irregularity in the output light of the light source device in the subscanning direction is neglected. Such a problem may be eliminated by stabilizing the quantity of light issuing from the light source or by illuminating a transmissive document uniformly. Nevertheless, when the light source is implemented by a fluorescent lamp, for example, it is necessary to provide extra devices for controlling the current to be applied to the lamp and the temperature of the lamp, a reflector having an accurate configuration, etc. This results in an increase in cost and, yet, incomplete compensation.